1. Technical Field
The present invention relates to a short-wavelength light source for converting a fundamental wave into a second harmonic using a wavelength conversion element.
2. Description of the Background Art
In the generation of a second harmonic utilizing a nonlinear optical effect, a coherent light as a fundamental wave from a fundamental wave light source can be converted into a second harmonic having a half wavelength. In this wavelength conversion, conversion efficiency can be improved and an high-output short wavelength light can be generated by an increase in the length of the wavelength conversion element or an increase in the power of the incident fundamental wave in the case of noncritical phase matching.
Here, in the wavelength conversion element using KTP as a nonlinear optical crystal, an increase in the power density of a fundamental wave or a second harmonic causes the generation of a color center called a gray track, whereby the fundamental wave and the second harmonic are absorbed. Upon the occurrence of the gray track, a local heat distribution is caused by absorption and the nonlinear optical crystal is destructively damaged. As a method for solving this, a method for dividing a crystal to obtain divided second harmonics is proposed in Japanese Unexamined Patent Publication No. H11-271823.
FIG. 29 is a construction diagram of a conventional short-wavelength light source. In FIG. 29, a part of a fundamental wave emitted from a fundamental wave light source 301 is wavelength-converted into a second harmonic H1 by a KTP crystal 302, thereby being separated into a fundamental wave F1 and the second harmonic H1 by a wavelength separating mirror 304. A part of the fundamental wave F1 is wavelength-converted into a second harmonic H2 by a next KTP crystal 303, and separated into a fundamental wave F2 and a second harmonic H2 by a wavelength separating mirror 305. In this way, by extracting the second harmonics in such a state where the power densities of the generated second harmonics do not exceed a specific value using a plurality of KTP crystals, efficiency is improved.
Japanese Unexamined Patent Publication No. 2007-94424 discloses a method by which a plurality of nonlinear optical crystals are arranged in a direction of a fundamental wave beam and the fundamental wave beam is incident on the respective nonlinear optical crystals via condensing optical systems for wavelength conversion.
Japanese Unexamined Patent Publication No. 2002-350914 discloses a method by which a plurality of nonlinear optical crystals are arranged in a direction of a fundamental wave beam and the fundamental wave beam is incident on the respective nonlinear optical crystals via condensing optical systems for wavelength conversion by the generation of a second harmonic or a sum frequency.
However, since the above conventional constructions require a plurality of nonlinear optical crystals and the second harmonics are emitted from different parts, a complicated optical system is required to use the second harmonics while multiplexing them, which has presented a problem of enlarging the light source. Further, since the output extracted from one nonlinear optical crystal is limited, many crystals are necessary to obtain a high output, which has caused a problem of making miniaturization and lower cost difficult. Since the maximum emission quantity of each beam is limited, there has been a problem of being difficult to realize a high-output single beam.
In the construction for wavelength-converting the fundamental wave beam by a plurality of wavelength conversion elements while condensing the fundamental wave beam by the condensing optical systems, there has been proposed a method for reducing the interference of beams generated by the wavelength conversion elements by rotating the polarizations of the beams generated by the respective wavelength conversion elements. However, there has been a problem of reducing conversion efficiency since the polarizations of the second harmonics generated by the respective wavelength conversion elements differ. Further, in the construction for causing the fundamental wave laser to be incident on different crystals and generating a short-wavelength light by the generation of a second harmonic or a sum frequency, the generation of a high-output second harmonic using a plurality of wavelength conversion elements is not considered.